1. INTRODUCTION:

The substitution reactions of the oxalato complexes of chromium(III) have been reviewed by Krishnamurthy¹. However, a few papers have been published on the kinetics of Cr(C₂O₄)₂X₂^n- moiety; where X is any monodentate ligand other than water^2-6.

Amino acids are organic molecules that constitute the most important part of biological structure and body chemistry. In the amino acids, the amino group is basic portion of the molecule and it is capable of reacting with both organic and inorganic acids to form salts, amide; while the acid proton is capable of reacting with bases. At isoelectric p^H, the amino acid exists as a neutral species and as an ion pair. The derivatives of amino acids are generally hormones or chemotherapeutical reagents. The study of the reactions of amino acids serves as a model to understand the complex nature of the biological macromolecules in the metabolic process.

Several workers^7-11have reported on the study of chromium(III) complexes as deficiency of chromium(III) ion results in an impairment of intravenous glucose tolerance and diabetics like symptoms in man and animals.

Chromium(III) metabolism is disrupted in diabetics, and its level in urine is double than that of in normal subjects¹². Clinical trials, though limited, have shown favorable response to chromium(III) administration. The biological and nutritional role of chromium in plants and animals is a topic of current interest.

Banerjea and Dutta Choudary ¹³ identified that the bond formation by glycine takes place simultaneously with the rupture of the Cr-OH₂ bond in the kinetics of reaction of hexaaquochromium(III) with glycine. Khan and Kabir-ud-din¹⁴ reinvestigated this reaction and suggested an associative interchange mechanism with a kinetic evidence for the formation of ion pair. In the pH range (3.0 to 3.8) tetra aquaglycinato chromium(III) complex¹⁵ is formed in the reaction between glycine and chromium(III) . In the anation study of mono (oxalato)tetraquochromium(III) with glycine, a glycine dependent path and glycine independent path representing an associative interchange(I_a)mechanism and dissociative inter change(I_d) mechanism is suggested by Subrahmanyam and Ananta Ramam¹⁶. A dissociate mechanism is proposed by Ramasami¹⁷ in the kinetics of Cr(NH₃)₅H₂O³⁺ - glycine reaction; Cr-OH₂ bond breaking is a dominant factor in this investigation.

Niogy and De¹⁸ noticed that the rate of anation by alanine is much faster than that of isotopic water exchange and other anation processes, in the kinetics of substitution of aquo ligand from hydroxopentaaquochromium(III) by DL-alanine and also studied the anation reaction with DL-phenylalanine¹⁹. Mitra and De^{20, 21} observed both bond breaking and bond making are significant and proposed an I_a mechanism in the kinetics of anation reaction of cis-diaquo-bis oxalatochromate(III) ion by DL-alanine and DL-phenylalanine .

Several workers^22-26 have reported the studies of kinetic reactions of hexaaquochromium(III) with different amino acids. Subba Rao et al. proposed²⁷ the amino acid dependent and amino acid independent paths in the reaction of kinetics of substitution of cis-bis(malonato) diaquochromate(III) with glycine, Dl- alanine and DL- phenyl alanine in acid medium. They have also²⁸ observed the amino acid dependent and amino acid independent paths in the kinetics of substitution of cis-bis(oxalato)diaquochromate(III) with amino acids such as glycine, DL-alanine and Dl- phenyl alanine in acid medium. In addition to Subba Rao et al. proposed²⁹the amino acid dependent and amino acid independent paths in the reaction of kinetics of substitution of cis-bis(malonato)diaquochromate(III) with glycine, Dl- alanine and DL- phenyl alanine in alkaline medium,

Basing on the available reported literature the following reactions are taking up in our laboratory. In the present communication, the results relating to the formation of chromium(III)-amino acid complex from cis-bis(oxalato)diaquochromate(III) and amino acids such as glycine, Dl-alanine and DL- phenylalanine in alkaline medium are reported .

2. EXPERIMENTAL:

The potassium salt of cis–bis(oxalato)diaquochromate(III) is prepared by reported³⁰ method and its purity is confirmed by analysis³¹ . The complex is analyzed for chromium and oxalate. The complex is decomposed with hot sodium hydroxide solution and then heated to boiling for a few minutes. Then it is oxidized with alkaline hydrogen peroxide to chromium (VI), which is estimated iodometrically. Oxalate is determined permangometrically after decomposing the complex with hot sodium hydroxide solution, filtering off the Cr(OH)₃ and acidifying the filtrate with H₂SO₄ .The oxalate/chromium ratio is found to be 2.04. Other method is also followed to confirm the purity of the synthesized complex. The visible absorption spectrum of the synthesized metal complex shows maxima at 415nm (ε_obsd = 67.5 dm³mol^-1cm^-1) and 560nm (ε_obsd = 50.5 dm³mol^-1cm^-1) as against reported³² maxima at 415nm (ε_reported = 66.0 dm³mol^-1cm^-1) and 560nm (ε_reported = 50.8 dm³mol^-1cm^-1). All other chemicals used are of reagent grade. Triple distilled water is used to prepare all the solutions. The product of the reaction between the substrate complex and aminoacids (Glycine, DL-alanine and DL-phenylalanine) are prepared by mixing different molar ratios of reactants viz., 1:1, 1:2 and 1:3 at pH 8.50 and thermo stating the mixture at 45 ^◦C for 36 hours. The absorption spectra of the resultant solution are recorded using an aqueous ligand solution of appropriate molarity in the reference cell and it is found that all the three product complexes are identical, having a maximum absorption at 398 and 540nm. The spectra difference between the product complex and substrate complex has been shown in Figure 1.

2.1 Kinetic studies:

The progress of the reaction is monitored by the absorbance measurements for the product at different intervals of time with a MILTON ROY Spectronic 1201 UV visible spectro photometer (U.S.A). Temperature control in these studies is achieved by the use of thermostat with circulating pump attachment enabling the control of the reaction vessel temperature ± 0.1 ^◦C. Measurement of pH are carried out with the help of a Systronics digital pH meter (model 335, INDIA ) with an accuracy of ± 0.01 .The pH is adjusted by adding dilute solution of sodium hydroxide. The ionic strength of the reaction medium is adjusted by addition of sodium perchlorate. Required quantities of amino acid (using the stock solutions of 0.25 mol dm^-3 of glycine and DL-alanine and 0.1 mol dm^-3 of DL-phenyl alanine) and sodium perchlorate( using 4.0 mol dm^-3 stock) solutions are taken in to a beaker and then pH is adjusted(to a particular value) and are taken into a stoppered containers and they are thermo stated to attain the experimental temperature, and also the substrate (metal complex) and distilled water is kept at that temperature. A known quantity of the metal complex (from 0.05 mol dm^-3 stock) solution is added and contents are diluted to a definite volume with distilled water. Conventional mixing technique is followed and a portion of the reaction mixture is transferred to an optical cell placed in the cell compartment of spectrophotometer. The rate of the reaction is monitored by measuring the absorbance at 530nm. The reaction was followed up to 80-90% completion. The absorbance reading at infinite time A_¥ is obtained, keeping the reaction mixture long enough for the reaction to be complete. The rate constants are evaluated by using software KINTOB^{33, 34} procedures. The substitution reaction has been studied as a function of the concentration of amino acids, pH and substrate. The kinetic runs are also performed at different temperatures.

3. RESULT AND DISCUSSIONS:

3.1 Effect of concentration of amino acids:

The Pseudo-First order rate constants are evaluated from the straight line plot of log (A_¥-A_t) vs. time by using KINTOB software procedure^.The rate of the reaction increases with increase in the concentration of amino acids (Table 1). In the amino acid variation studies the pH is kept constant by the addition of sodium hydroxide. A typical plot of k_obs vs. [Glycine] (Figure 2) is linear with an intercept on the rate axis, suggesting that the reaction proceeds by two paths: amino acid dependent and amino acid independent. The same types of plots are also observed in the case of DL-alanine and DL-phenyl alanine.

3.1.1 Effect of pH:

The concentration of substrate and amino acid are kept constant and the pH is varied. The rate of the reaction increases with increase in pH (Table 2). A plot of pK vs. pH gives a straight line with a slope less than unity indicating the involvement of H⁺ in the equilibrium step.

Table 1 Variation of rate constants with [Glycine], [DL-alanine] and [DL-Phenyl alanine]

With [Glycine] [Cr(OX)₂(H₂O)₂^-] = 4.0 ´ 10^-3 mol dm^-3 pH = 8.50, m = 1.0 mol dm^-3				With [DL-alanine] [Cr(OX)₂(H₂O)₂^-] = 4.0 ´ 10^-3 mol dm^-3 pH = 8.50, m = 1.0 mol dm^-3				With [DL-phenylalanine] [Cr(OX)₂(H₂O)₂^-] = 2.0 ´ 10^-3 mol dm^-3 pH = 8.50, m = 1.0 mol dm^-3
[Glycine] ´10²(mol dm^-3)	Rate/k_obs ´ 10^-3 s^-1			[DL- alanine] ´ 10² (mol dm^-3)	Rate/k_obs ´ 10^-3 s^-1			[DL-phenylalanine] ´ 10² (mol dm^-3)	Rate/k_obs ´ 10^-3 s^-1
[Glycine] ´10²(mol dm^-3)	30.0 ^◦C	35.0 ^◦C	40.0 ^◦C	[DL- alanine] ´ 10² (mol dm^-3)	30.0 ^◦C	35.0 ^◦C	40.0 ^◦C	[DL-phenylalanine] ´ 10² (mol dm^-3)	35.0 ^◦C	40.0 ^◦C	45.0 ^◦C
2.0	0.41	0.81	1.17	2.0	0.32	0.68	1.02	1.0	0.59	0.94	1.28
4.0	0.61	1.05	1.44	4.0	0.42	0.84	1.25	1.5	0.66	1.02	1.43
6.0	0.79	1.30	1.77	6.0	0.55	0.99	1.45	2.0	0.73	1.11	1.59
8.0	1.01	1.55	2.08	8.0	0.65	1.17	1.69	2.5	0.83	1.20	1.73
10.0	1.21	1.80	2.41	10.0	0.79	1.34	1.92	3.0	0.90	1.29	1.91
12.0	1.40	2.05	2.70	12.0	0.91	1.49	2.17	---	---	---

Table 2 Variation of rate constants with pH

[Cr(OX)₂(H₂O)₂^-] = 4.0 ´ 10^-3 mol dm^-3 [Glycine] = 8.0 ´ 10^-2 mol dm^-3 m = 1.0 mol dm^-3				[Cr(OX)₂(H₂O)₂^-] = 4.0 ´ 10^-3 mol dm^-3 [DL-alanine] = 8.0 ´ 10^-2 mol dm^-3 m = 1.0 mol dm^-3				[Cr(OX)₂(H₂O)₂^-] = 2.0 ´ 10^-3 mol dm^-3 [DL-phenylalanine] = 2.0´10^-2 mol dm^-3 m = 1.0 mol dm^-3
pH	Rate/k_obs ´ 10^-3 s^-1			pH	Rate/k_obs ´ 10^-3 s^-1			pH	Rate/k_obs ´ 10^-3 s^-1
pH	30.0 ^◦C	35.0 ^◦C	40.0 ^◦C	pH	30.0 ^◦C	35.0 ^◦C	40.0 ^◦C	pH	35.0 ^◦C	40.0 ^◦C	45.0 ^◦C
7.40	0.62	1.07	1.30	7.40	0.38	0.78	1.04	7.50	0.36	0.55	0.76
7.80	0.73	1.25	1.55	7.60	0.46	0.84	1.18	7.80	0.45	0.67	0.95
8.10	0.80	1.34	1.70	8.00	0.52	0.93	1.31	8.10	0.53	0.83	1.15
8.20	0.87	1.42	1.83	8.30	0.57	1.06	1.49	8.30	0.62	0.98	1.36
8.40	0.97	1.53	2.03	8.45	0.63	1.17	1.66	8.50	0.73	1.11	1.59
8.55	1.05	1.59	2.11	8.60	0.69	1.22	1.74	8.60	0.79	1.19	1.73
8.70	1.30	1.82	2.40	8.70	0.74	1.29	1.86	8.80	0.91	1.37	1.98

3.1.2 Effect of substrate:

In this kinetic study at pH 8.50, keeping the constant concentration of amino acids (Glycine, DL-alanine and DL-phenyl alanine) and varying the concentration of metal complex, the rate constants are evaluated. From this it can be observed that the concentration of the substrate (metal complex) does not have any effect on the rate constant, showing thereby the order with respect to substrate is unity.

4. Spectrophotometric studies:

When cis-bis(oxalato)diaquochromate(III) is taken and different amounts of sodium hydroxide are added, the solution retains its bluish voilet colour till the pH value reaches to 5.00. Beyond this pH the solution turns to different shade of green. Spectra of cis-bis(oxalato)diaquochromate(III) [0.004 mol dm^-3 ] recorded at different pH values by adding different quantities of 0.01 mol dm^-3 NaOH solution . From the Figure 3, it can be noted that the peak at longer wavelength exhibits a bathochromic shift with an increase in molar extinction coefficient. The peak at shorter wavelength is not altered, but the molar extinction coefficient increases with increasing up to 10.95. However at pH 12.00, the molar extinction coefficient decreases. These observations indicate the gradual formation of species of the type Cr(OX)₂(H₂O)(OH)^2- and Cr(OX)₂ (OH)₂^3-/or Cr(OX)(-OX)(OH)₂(H₂O)^3-.

In the octahedral substitution reactions mainly two important effects operate viz., cis and trans effect. Under the experimental conditions the cis effect operates in the pH range 5.40 – 6.80 and 7.40-8.80. On the other hand trans effect predominates at higher pH (> 11.00). Under the experimental conditions the amino acids exists partly as zwitter ion³⁵ and in an unprotonated form of the type, AA^-. Basing on the experimental observations the following mechanism is proposed for the substitution process in alkaline medium.

Proposed mechanism:

Amino acid (AA) dependent path:

Rate determining

Where AA^- represents Amino Acid (Glycine / DL-alanine / DL-phenylalanine).

Amino acid (AA) independent path:

Where (-OX) represents single ended oxalate (In ligand independent path a single ended oxalate is envisaged³⁶).

Rate = k₁ [Cr(OX)₂(OH)₂^3-] [AA^-] + k₂[Cr(OX)₂(H₂O)(OH)^2-] -----( 5)

But K =

On substituting

K[Cr(OX)₂(H₂O)(OH)^2-][OH^-] for [Cr(OX)₂(OH)₂^3-] in equation (5)

Rate = Kk₁[Cr(OX)₂ (H₂O)(OH)^2-] [OH^- ] [AA^- ] + k₂[Cr(OX)₂(H₂O)(OH)^2-] ………………...(6)

= k_obs

k_obs = K k₁ [AA^-] [OH^-] + k₂ ………. (7)

At fixed [OH^-] i.e. at a constant pH the equation transforms to

Rate = Kk₁ [AA^-] + k₂

= k'[AA^-] + k'' ……….. (8)

Where k' = Kk₁ and k'' = k₂ (for convenience k₂ is represented as k''). From the equation (8) it can be seen that when a plot of observed rate constant vs. the concentration of the ligand is prepared at a constant pH, the slope will be k' and the intercept will be k''.

Substitution reactions at Chromium(III) centers are inert and generally proceed by associative mechanism(A), interchange associative(Ia), Id and D-mechanism based on the experimental conditions. Substitution reactions at chromium(III) proceed by a hydrolysis pathway as well as direct substitution. As a result, two routes for these reactions are reported: ligand dependent and ligand independent routes. The OH group present in the substrate species in alkaline medium functions as a cis-activator facilitating substitution up to a pH of 8.80. There is a possibility that the Cr(III) species become unreactive and not eminable for substitution beyond this pH. Hence, a mechanism is proposed where a single ended oxalate species forms as an intermediate in the amino acid independent pathway. This is a clear example of a reactionproceeding by an Ia and D-mechanism.

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Received on 25.08.2011 Modified on 04.09.2011

Asian J. Research Chem. 4(10): Oct., 2011; Page 1612-1615